Expanded polystyrene, commonly known as Styrofoam, is a lightweight material used widely in packaging and insulation. Its common use leads to questions about its resistance to household chemicals, especially solvents like alcohol. Determining if alcohol “dissolves” this material requires a closer look at the chemistry of both substances. The answer is a nuanced explanation tied to molecular interactions.
Understanding Polystyrene Structure
The material commonly called Styrofoam is Expanded Polystyrene (EPS), a plastic foam derived from the polymer polystyrene. Polystyrene is a long-chain synthetic hydrocarbon, consisting of repeating units of the styrene monomer. Since it is built almost entirely from carbon and hydrogen atoms, the polymer chain is highly non-polar.
This molecular structure gives polystyrene its resistance to many liquids. EPS is an extremely low-density material, consisting of approximately 95% to 98% trapped air. This cellular structure provides its insulating and cushioning properties. Any physical change in the material’s bulk is primarily due to the loss of air, not the disappearance of plastic.
The Principle of Chemical Dissolution
The interaction between any solvent and a substance like polystyrene is governed by the fundamental chemical concept: “like dissolves like.” This principle states that a substance will dissolve another substance that shares a similar polarity. Polarity refers to the distribution of electrical charge within a molecule.
Water is a classic example of a polar solvent, which is why it dissolves other polar substances like sugar. Non-polar substances, such as oils or gasoline, lack this uneven charge distribution and are only soluble in other non-polar solvents. For a true chemical dissolution to occur, the solvent molecules must overcome the attractive forces holding the polymer chains together and completely separate them.
How Common Alcohols Affect Styrofoam
Common household alcohols, such as ethanol and isopropyl alcohol, fall into an intermediate category of polarity. They contain a non-polar hydrocarbon chain but also a polar hydroxyl (-OH) group. This structure makes them less non-polar than the polystyrene they are attempting to dissolve. Due to this difference in polarity, these alcohols are not strong enough to break down the long, highly non-polar polystyrene chains efficiently.
When Styrofoam is exposed to these alcohols, the effect is typically minimal, often categorized as “limited resistance.” Isopropyl alcohol may cause slight softening, minor swelling, or a small reduction in volume, but this is not true chemical dissolution. The small amount of plastic material remains; the observed volume reduction is mostly the result of the alcohol slowly diffusing into the small plastic struts and displacing the trapped air. Over a long period, high-concentration alcohol may cause some physical degradation, but it does not rapidly melt the foam into a liquid.
Solvents That Achieve True Dissolution
In contrast to alcohol, many other liquids can achieve the true, rapid chemical dissolution of polystyrene. These highly effective solvents are far more non-polar, closely matching the chemical nature of the polystyrene polymer. Acetone, a common ingredient in nail polish remover, is a well-known example that dissolves Styrofoam almost instantly.
Other powerful non-polar solvents include toluene, xylene, and gasoline, which contain hydrocarbon molecules that easily penetrate and separate the polystyrene chains. When Styrofoam is placed in one of these liquids, the solvent quickly breaks the weak intermolecular forces holding the foam structure together. This process releases the vast amount of trapped air, leaving behind a thick, viscous liquid that is essentially the concentrated polystyrene polymer dissolved in the solvent. These strong solvents reinforce the “like dissolves like” rule.